The present invention relates generally to encapsulated devices, and more particularly to encapsulated plasma sensitive devices, and to methods of making encapsulated plasma sensitive devices.
Many devices are subject to degradation caused by permeation of environmental gases or liquids, such as oxygen and water vapor in the atmosphere or chemicals used in the processing of the electronic product. The devices are usually encapsulated in order to prevent degradation.
Various types of encapsulated devices are known. For example, U.S. Pat. No. 6,268,695, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making,” issued Jul. 31, 2001; U.S. Pat. No. 6,522,067, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making,” issued Feb. 18, 2003; and U.S. Pat. No. 6,570,325, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making”, issued May 27, 2003, all of which are incorporated herein by reference, describe encapsulated organic light emitting devices (OLEDs). U.S. Pat. No. 6,573,652, entitled “Encapsulated Display Devices”, issued Jun. 3, 2003, which is incorporated herein by reference, describes encapsulated liquid crystal displays (LCDs), light emitting diodes (LEDs), light emitting polymers (LEPs), electronic signage using electrophoretic inks, electroluminescent devices (EDs), and phosphorescent devices. U.S. Pat. No. 6,548,912, entitled “Semiconductor Passivation Using Barrier Coatings,” issued Apr. 15, 2003, which is incorporated herein by reference, describes encapsulated microelectronic devices, including integrated circuits, charge coupled devices, light emitting diodes, light emitting polymers, organic light emitting devices, metal sensor pads, micro-disk lasers, electrochromic devices, photochromic devices, microelectromechanical systems, and solar cells.
Generally, encapsulated devices can be made by depositing barrier stacks adjacent to one or both sides of the device. The barrier stacks typically include at least one barrier layer and at least one decoupling layer. There could be one decoupling layer and one barrier layer, there could be multiple decoupling layers on one side of one or more barrier layers, or there could be one or more decoupling layers on both sides of one or more barrier layers. The important feature is that the barrier stack has at least one decoupling layer and at least one barrier layer.
One embodiment of an encapsulated display device is shown in FIG. 1. The encapsulated display device 100 includes a substrate 105, a display device 110, and a barrier stack 115. The barrier stack 115 includes a barrier layer 120 and a decoupling layer 125. The barrier stack 115 encapsulates the display device 110, preventing environmental oxygen and water vapor from degrading the display device.
The barrier layers and decoupling layers in the barrier stack can be made of the same material or of a different material. The barrier layers are typically about 100-400 Å thick, and the decoupling layers are typically about 1000-10,000 Å thick.
Although only one barrier stack is shown in FIG. 1, the number of barrier stacks is not limited. The number of barrier stacks needed depends on the level of water vapor and oxygen permeation resistance needed for the particular application. One or two barrier stacks should provide sufficient barrier properties for some applications. The most stringent applications may require five or more barrier stacks.
The barrier layers can be deposited using a vacuum process, such as sputtering, chemical vapor deposition (CVD), metalorganic chemical vapor deposition (MOCVD), plasma enhanced chemical vapor deposition (PECVD), evaporation, sublimation, electron cyclotron resonance-plasma enhanced vapor deposition (ECR-PECVD), and combinations thereof. Suitable barrier materials include, but are not limited to, metals, metal oxides, metal nitrides, metal carbides, metal oxynitrides, metal oxyborides, and combinations thereof.
The decoupling layers can be deposited using a vacuum process, such as flash evaporation with in situ polymerization under vacuum, or plasma deposition and polymerization, or atmospheric processes, such as spin coating, ink jet printing, screen printing, or spraying. Suitable materials for the decoupling layer include, but are not limited to, organic polymers, inorganic polymers, organometallic polymers, hybrid organic/inorganic polymer systems, and silicates.
As an example, an OLED can be encapsulated with a barrier stack including one or more polymeric decoupling layers and one or more barrier layers. The polymeric decoupling layers can be formed from acrylate functional precursors which are deposited using flash evaporation and polymerized by ultraviolet (UV) exposure. The barrier layers can be reactively sputtered aluminum oxide.
Depositing multi-layer barrier stacks on relatively insensitive substrates such polymer films does not typically result in damage to the substrate. In fact, several patents disclose the use of plasma treatment to improve properties for a multi-layer barrier on a substrate. U.S. Pat. No. 6,083,628 discloses plasma treatment of polymeric film substrates and polymeric layers from acrylates deposited using a flash evaporation process as a means of improving properties. U.S. Pat. No. 5,440,466 similarly discusses plasma treatment of substrates and acrylate layers to improve properties.
However, we have found that some of the devices being encapsulated have been damaged by the plasma used in depositing the barrier and/or decoupling layers. Plasma damage has occurred when a substrate with a plasma sensitive device on it, such as an OLED, is encapsulated with a multi-layer barrier stack in which a plasma based and/or assisted process is used to deposit a barrier layer or decoupling layer. For example, plasma damage has occurred when reactively sputtering a barrier layer of AlOx under conditions suitable for achieving barrier properties, sputtering a barrier layer of AlOx onto the top surface of a plasma sensitive device, and/or sputtering a barrier layer of AlOx on a vacuum deposited, acrylate based polymeric layer. The damage observed when depositing a barrier layer onto a previously deposited decoupling layer is distinct, and is the subject of co-pending application Ser. No. 60/711,136 (VIT 0062 MA).
Plasma damage associated with deposition of a barrier layer, a decoupling layer, or another layer essentially has a negative impact on the electrical and/or luminescent characteristics of a device resulting from encapsulation. The effects will vary by the type of device, the manufacturer of the device, and the wavelength of the light emitted. It is important to note that plasma damage is dependent on the design of the device to be encapsulated. For example, OLEDs made by some manufacturers show little to no plasma damage while OLEDs made by other manufacturers show significant plasma damage under the same deposition conditions. This suggests that that there are features within the device that affect its sensitivity to plasma exposure.
One way to detect plasma damage is to measure the voltage needed to achieve a specified level of luminescence. Another way is to measure the intensity of the luminescence. Plasma damage results in higher voltage requirements to achieve the same level of luminescence (typically 0.2 to 0.5 V higher for an OLED), and/or lower luminescence.
Although not wishing to be bound by theory, plasma damage that is observed when a decoupling layer employing plasma, a sputtered AlOx, or another layer employing plasma is formed (deposited) directly on an OLED or other sensitive device is believed to be due to an adverse interaction with one or more components of the plasma, including charged or neutral species, UV radiation, and high thermal input.
Thus, there is a need for a method of preventing the damage caused by processes utilizing plasma in the encapsulation of various devices.